King Abijah said God gave Israel's kingship to David's descendants forever by a covenant of what?

Answer: Salt

From Mount Zemaraim Abijah reminded Jeroboam's army that the LORD gave the kingdom to David and his sons by a covenant of salt.

“Then Abijah stood on Mount Zemaraim in the hill country of Ephraim and said, “Hear me, O Jeroboam and all Israel! Do you not know that the LORD, the God of Israel, has given the kingship of Israel to David and his descendants forever by a covenant of salt?”— 2 Chronicles 13:4-5 (BSB)